(1) Field of the Invention
The present invention relates to a semiconductor laser device and a manufacturing method thereof. More particularly, the present invention relates to a monolithic dual-wavelength semiconductor laser device having two semiconductor lasers of mutually different oscillation wavelengths, and to a manufacturing method thereof.
(2) Description of the Related Art
In recent years, large capacity DVD drives for recording and playback of optical information are rapidly becoming common in various fields, most notably in the field of video players. It is strongly desired that DVD drives be capable of reading conventional recording media, such as CDs, CD-Rs, and CD-RWs. To satisfy the demand, a DVD drive has two light sources for an optical pickup to record and play back DVDs and CDs. The one used for DVDs is a red semiconductor laser that emits light at wavelengths around 650 nm. The one used for CDs is an infrared semiconductor laser that emits light at wavelengths around 780 nm.
With the trend toward smaller information processing devices such as PCs, recording/playback devices for DVDs and other recording media are also required to be more compact and slim. To this end, it is essential to make optical pickups smaller and thinner. An optical pickup may be made smaller and thinner by reducing the number of optical components to simplify the structure. One scheme which helps to reduce the number of optical components is to integrate a red semiconductor laser and an infrared semiconductor laser into a single piece.
As one conventionally known example, JP Patent Application Publication No. 11-186651 (hereinafter, “patent literature 1”) suggests a monolithic semiconductor laser device having red and infrared semiconductor lasers integrated on a single semiconductor substrate. In addition to the advantage that the two semiconductor lasers are integrated into a single piece, the disclosed construction allows such optical components as a collimator lens and a beam splitter to be shared between the red and infrared semiconductor lasers. Thus, the disclosed construction helps to reduce the device size and thickness.
Regarding such a monolithic semiconductor laser device, it is desired to improve the light output while ensuring stability and reliability of the device at high output power operation. For this purpose, more and more devices have started to employ a real refractive index guide structure and a window structure formed at a laser facet. In the window structure, the bandgap near the laser facet is larger than the bandgap of laser beam emission. In order to improve laser output, a greater amount of current needs to be supplied. With the increase in supply current, the vicinity of laser facet is subjected to heat generated through nonradiative recombination resulting from the interface state existing between the facet coat film and the laser facet. Because of the heat, the laser is more prone to deterioration. By employing the laser facet window structure, however, laser deterioration by heat is suppressed.
Since both the infrared and red lasers are expected to be operated at high power, both the lasers need to have facet window structures.
Some manufacturing methods known in the art are disclosed in, for example, JP patent application publication Nos. 2001-210907, 2002-026447, and 2001-345514 (hereinafter, referred to as the “patent literatures 2, 3, and 4” in the stated order).
FIG. 7 illustrate the manufacturing method of a red laser device having facet window structures, which is disclosed in the patent literature 2.
As shown in FIG. 7A, the following layers are epitaxially grown on an n-type semiconductor substrate 401 made of GaAs in the stated order: an n-type buffer layer 402 made of GaAs; an n-type cladding layer 403 made of AlGaInP; an active layer (a multiple quantum well structure with the oscillation wavelength of 660 nm) 404; a p-type first cladding layer 405 made of AlGaInP; an etching stop layer 406 made of GaInP; a p-type second cladding layer 407 made of AlGaInP; a p-type intermediate layer 408 made of GaInP; and a p-type contact layer 409 made of GaAs.
Next, with the use of a film forming device such as a sputtering device, ZnO is deposited on the entire wafer surface to form a ZnO film 410 (not illustrated). The ZnO film 410 is then patterned using photoresist, so as to leave the ZnO film only in the vicinities of later-formed laser facets (the remaining regions of the ZnO film are denoted by the reference numeral 410a).
Next, an insulating film 411 is deposited on the entire wafer surface. Through a thermal treatment, Zn diffuses from each ZnO film 410a into the laminated semiconductor layers. The thermal treatment is conducted with temperatures and time appropriate for Zn to reach the active layer (FIG. 7B).
In a region into which Zn has been diffused, the active layer 404 undergoes structural disorder. As a result, a window structure 412 having a larger bandgap than that of the active layer 404 is formed. Finally, each ZnO film 410a is removed (FIG. 7C).
As is generally known, GaAs materials are smaller in Zn diffusion coefficient than AlGaInP materials. With the use of this property, the GaAs contact layer 409 acts as a Zn diffusion controlling layer in the Zn diffusion process, so that a window structure is stably formed at each facet. In addition, excessive Zn diffusion in the window structure 412 is suppressed. This is advantages in a subsequent step of processing the p-type second cladding layer 407 into a striped pattern, because crumbling of the GaInP etching stop layer 406 existing below the p-type second cladding layer 406 is prevented. Consequently, the stripped pattern conforming in shape to the laser gain regions can be formed.
However, as stated above, the Zn diffusion coefficient of GaAs materials is rather small. Since the infrared laser has a GaAs-based active layer, it is more difficult to form a window structure than in a red laser having an AlGaInP-based active layer. This limitation may be addressed by separately subjecting the infrared and red lasers to different thermal treatments to achieve the substantially equal levels of Zn diffusion. Yet, by separately performing thermal treatments, a laser treated first for window structure formation is subjected to unnecessary heat at the time of conducting a thermal treatment for the other laser. The excessive heat induces occurrence of defects in the semiconductor. In addition, Zn excessively diffuses to reduce reliability of the active layer at the laser gain region.
Turning now to the patent literature 3, it is disclosed that AlGaAs instead of GaAs is used for a p-type contact layer to facilitate Zn diffusion. As a result, even in an infrared laser using a GaAs material, window structures are formed with good controllability and high reproducibility.
Turning now to the patent literature 4, it is disclosed that the active layers of the infrared and red lasers are individually optimized in thickness. The active layers each having an optimized thickness allows Zn to be suitably diffused by a single thermal treatment to from window structures in both the lasers.